Image-guided radiation treatment with imaging data using imaging radiation at different energy levels

ABSTRACT

A method of image-guided radiation treatment is described. The method may include acquiring a pre-treatment image of a patient and generating a first set of image data of part or all of the patient using imaging radiation at a first energy level and a second set of image data of part or all of the patient using imaging radiation at a second energy level. The method may also include processing the first and second sets of image data to generate an enhanced image, wherein the enhanced image comprises a combination of the first and second sets of image data, and wherein part or all of the image data comprises the target. The method may also include registering the enhanced image with the pre-treatment image to obtain a registration result and tracking movement and position of the target using the registration result to generate tracking information.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/679,686, filed on Nov. 11, 2019, which is a continuation of U.S.patent application Ser. No. 15/208,049, filed on Jul. 12, 2016, nowissued as U.S. Pat. No. 10,485,989 on Nov. 26, 2019, which is acontinuation of U.S. patent application Ser. No. 13/536,737, filed onJun. 28, 2012, now issued as U.S. Pat. No. 9,415,240 on Aug. 16, 2016,which claims the priority to U.S. Provisional Patent Application No.61/550,309, filed Oct. 21, 2011, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

This application relates to apparatus for generating multi-energy x-rayimages and systems and methods for using multi-energy x-ray images inimage-guided radiation therapy.

BACKGROUND

Oncology is the branch of medicine directed to the study of thedevelopment, diagnosis, treatment, and prevention of tumors. A tumor isan abnormal growth of tissue serving no physiological function. A tumormay be malignant (cancerous) or benign. A malignant tumor may exhibituncontrolled, progressive multiplication of cells and spread cancerouscells to other parts of the body (metastasizes) through blood vessels orthe lymphatic system. A benign tumor does not metastasize, but can stillbe life-threatening if it impinges on critical body structures such asnerves, blood vessels, and organs.

Radiosurgery and radiotherapy are radiation treatment systems that useexternal radiation beams to treat tumors and other lesions by deliveringa prescribed dose of radiation (e.g., x-rays, protons, or gamma rays) toa target volume (region of interest, or ROI) while minimizing radiationexposure to the surrounding tissue. The object of both radiosurgery andradiotherapy is the destruction of abnormal tissue while sparing healthytissue and critical structures. Radiotherapy is characterized by a lowradiation dose per treatment and many treatments (e.g., 30 to 45 days oftreatment). Radiosurgery is characterized by a relatively high radiationdose to a tumor in one, or at most a few, treatments. In bothradiotherapy and radiosurgery, the radiation dose is delivered to thetumor site from multiple angles. As the angle of each radiation beam isdifferent, each beam passes through a different area of healthy tissueon its way to the tumor. As a result, the cumulative radiation dose atthe tumor is high, while the average radiation dose to the surroundinghealthy tissue is low. Unless specified otherwise, radiosurgery andradiotherapy are used interchangeably in the present application.

Radiation treatment systems may be used together with an imaging systemfor image-guided radiation therapy (IGRT). The imaging system acquiresin-treatment images, e.g., x-ray, ultrasound, CT, or PET, that may beused to for patient set up and in some instances (e.g., AccurayIncorporated's CyberKnife® Radiosurgery System) guide the radiationdelivery procedure and track in-treatment target motion. Target motiontracking may be accomplished by correcting for differences in targetposition by acquiring and registering intratreatment images withreference images, known as digitally reconstructed radiographs (DRRs),rendered from a pre-treatment computed tomography (CT) scan, which mayotherwise be known as the treatment planning image.

Previously-known IGRT systems use imaging systems that generate singleenergy x-ray images during treatment. Such systems suffer, however, froma variety of drawbacks. For example, x-ray attenuation characteristicsare dependent on x-ray energy and thus a single energy x-ray image mayhave limited differentiation ability for certain materials. An x-rayimage generated using a low x-ray energy (e.g., ˜50-100 kV) will displaysignificant attenuation in soft tissue and radio-opaque objects such asskeletal structures, fiducials, and contrast agents. Conversely, anx-ray image generated using a high x-ray energy (e.g., ˜100 kV-6 MV,preferably 100-150 kV) will display less attenuation of soft tissue thana low x-ray energy image, but still significant attenuation inradio-opaque objects such as skeletal structures, contrast agents, andfiducials.

In view of the above-noted drawbacks of previously-known systems, itwould be desirable to provide apparatus and methods for generating x-rayimages with increased discrimination between soft tissue andradio-opaque objects.

It further would be desirable to provide systems and methods forprocessing and using such x-ray images in image-guided radiationtherapy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of a previously-known image-guidedradiation therapy (IGRT) system.

FIG. 2 illustrates a perspective view of an IGRT system and a schematicdiagram of a computer system in accordance with the principles of thepresent invention.

FIGS. 3A and 3B are, respectively, a side view and a plan view of anexemplary x-ray source filter for generating multi-energy x-ray images.

FIGS. 4A and 4B are, respectively, a side view and a plan view of analternative x-ray source filter for generating multi-energy x-rayimages.

FIG. 5A is a side view of yet another x-ray source filter for generatingmulti-energy x-ray images.

FIG. 5B is a plan view of the x-ray source filter of FIG. 5A at a firstposition.

FIG. 5C is a plan view of the x-ray source filter of FIG. 5A at a secondposition.

FIGS. 6A and 6B are, respectively, an exploded view and an assembledview of an exemplary x-ray detector filter assembly for generatingmulti-energy x-ray images.

FIG. 7 is a schematic view of an exemplary IGRT system for use withmulti-energy x-ray images.

FIGS. 8A and 8B are x-ray images generated by an exemplary real-timemulti-energy imaging system.

FIGS. 8C and 8D are x-rays images processed by an exemplary systemprocessor.

FIG. 9 illustrates an exemplary method for processing multi-energy x-rayimage data in IGRT.

FIG. 10 is a plot illustrating change in x-ray spectra after traversalthrough a filter for obtaining an optimal energy level.

FIG. 11 illustrates an exemplary method for registering multi-energyx-ray images in IGRT.

FIG. 12 illustrates an exemplary method for target tracking usingmulti-energy x-ray images in IGRT.

FIG. 13 illustrates an exemplary method for target tracking using amotion-edge artifact in IGRT.

FIGS. 14A and 14B are illustrations of x-ray images of a target at firstand second positions, respectively.

FIG. 14C is an illustration of a motion-edge artifact image generatedafter subtracting overlapping portions of the target at the first andsecond positions.

FIG. 15 illustrates a method for target tracking during respirationusing multi-energy x-ray images in IGRT.

DETAILED DESCRIPTION

This disclosure relates to apparatus for generating multi-energy x-rayimages and systems and methods for processing and using multi-energyx-ray images in image-guided radiation therapy. Advantageously, x-raysgenerated at an energy level within a low energy range (e.g., ˜50-100kV) may be enhanced with x-rays generated at an energy level within ahigh x-ray energy range (e.g., ˜100 kV-6 MV, preferably 100-150 kV) toprovide x-ray images having high definition for images of soft tissue,and radio-opaque objects such as skeletal structures, fiducials, andcontrast agents. Moreover, x-rays generated within the low energy rangemay be enhanced with x-rays generated within the high x-ray energy rangein a weighted manner to provide x-ray images without soft tissue or,alternatively, x-rays images without radio-opaque objects. The enhancedx-ray images may be used for enhanced target tracking and positioningfor image-guided radiation therapy.

FIG. 1 illustrates a perspective view of a previously-known image-guidedradiation therapy (IGRT) system 100. IGRT system 100 includesarticulated robot arm 102, MV radiation source 104, x-ray sources 106and 108, x-ray detectors 110 and 112, and treatment table 114. Anexample of IGRT system 100 is a CYBERKNIFE® Robotic Radiosurgery Systemavailable from Accuray, Incorporated, Sunnyvale, Calif.

In FIG. 1, MV radiation source 104 is configured to generate treatmentradiation beams, e.g., x-ray photon, electron, or proton beams, at atarget volume, e.g., a tumor, within a patient. MV radiation source 104generally includes a linear accelerator (LINAC). MV radiation source 104is mounted on the end of articulated robot arm 102 to provide multiple(e.g., 5 or more) degrees of freedom of motion in order to position MVradiation source 104 to irradiate tumorous tissue with highly-collimatedbeams delivered from many angles in an operating volume (e.g., sphere)around the patient. Treatment may involve beam paths with a singleisocenter, multiple isocenters, or with a non-isocentric approach (e.g.,the beams need only intersect with the targeted tumor mass and do notnecessarily converge on a single point, or isocenter, within the targetregion). Treatment can be delivered in either a single session(mono-fraction), in a small number of sessions (hypo-fractionation), orin a large number (30-40) of sessions (standard fractionation) asdetermined during treatment planning.

IGRT system 100 has a real-time imaging system that includes first x-raysource 106, second x-ray source 108, first x-ray detector 110, andsecond x-ray detector 112. First x-ray source 106 is paired with firstx-ray detector 110 to establish a first “channel” of a stereoscopicx-ray imaging system, and second x-ray source 108 is paired with secondx-ray detector 112 to establish a second “channel.” During radiationtreatment, first and second x-ray sources 106 and 108 emit single-energyx-ray imaging radiation that travels through a patient and treatmenttable 114 and is received by first and second x-ray detectors 110 and112, respectively. The in-treatment, single-energy x-ray images are usedto set up and guide the treatment radiation delivery procedure and trackin-treatment target motion. The target is tracked by correcting fordifferences in target position in the in-treatment, single-energy x-rayimages and registering them with reference images, known as digitallyreconstructed radiographs (DRRs), rendered from a pre-treatment computedtomography (CT) scan.

Applicants have concluded that higher definition in-treatment x-rayimages may be generated by providing apparatus for generatingmulti-energy x-ray images. Further, applicants have discovered systemsand methods for processing and using multi-energy x-ray images inimage-guided radiation therapy.

Apparatus For Generating Multi-Energy X-Ray Images

FIG. 2 illustrates a perspective view of image-guided radiation therapy(IGRT) system 200 and a schematic diagram of computer system 250 inaccordance with the principles of the present invention. IGRT system 200includes articulated robot arm 102, MV radiation source 104, x-raysources 206 and 208, x-ray detectors 210 and 212, and treatment table114. Articulated robot arm 102, MV radiation source 104, and treatmenttable 114 may be conventional and, thus, are not described in detail.

Advantageously, IGRT system 200 has a real-time imaging systemconfigured to generate multi-energy x-ray images. The real-time imagingsystem includes first x-ray source 206, second x-ray source 208, firstx-ray detector 210, and second x-ray detector 212. First x-ray source206 is paired with first x-ray detector 210 to establish a first“channel” of a stereoscopic x-ray imaging system, and second x-raysource 208 is paired with second x-ray detector 212 to establish asecond “channel.” During radiation treatment, first and second x-raysources 206 and 208 emit imaging radiation that travels through part orall of a patient and treatment table 114 and is received by first andsecond x-ray detectors 210 and 212, respectively.

First and second x-ray sources 206 and 208 may include kV x-ray tubetechnology, x-ray source array technology, broad energy technology, orother suitable x-ray source technology. X-ray source array refers to asource of x-rays comprising a plurality of spatially distinct,electronically controlled x-ray emitters or emission spots (focal spots)that are addressable on at least one of an individual or groupwise basissuch as an x-ray source array available from Triple Ring Technologies(Newark, Calif.) or XinRay Systems (Research Triangle Park, N.C.). Firstand second x-ray detectors 210 and 212 may be amorphous silicondetectors or other suitable detectors capable of producing high-qualitytwo-dimensional x-ray images during an IGRT. X-ray sources 206 and 208may be mounted in or near the ceiling of a treatment vault, while x-raydetectors 210 and 212 may be mounted in or near the floor of thetreatment vault as illustrated, although the scope of the preferredembodiments is not limited thereto. It should be noted that although thereal-time imaging system of IGRT system 200 is illustrated as having twox-ray sources and two x-ray detectors, IGRT system 200 is not limitedthereto. For example, IGRT system 200 may have one, three, four or morex-ray sources and one, three, four or more x-ray detectors.

In accordance with one embodiment, first x-ray source 206 is configuredto emit imaging radiation at an energy level within a low x-ray energy(e.g., ˜50-100 kV). An x-ray generated from low energy radiation hassignificant attenuation in soft tissue and radio-opaque objects and theresulting image will display soft tissue in conjunction withradio-opaque objects. In this embodiment, second x-ray source 208 isconfigured to emit radiation at an energy level within a high x-rayenergy range (e.g., ˜100-150 kV). An x-ray generated from high energyradiation has greater attenuation in radio-opaque objects, but littleattenuation in soft tissue, and the resulting image will displayprimarily radio-opaque objects without soft tissue.

After the emitted radiation travels through the patient, first x-raydetector 210 receives the low energy imaging radiation and second x-raydetector 212 receives the high energy imaging radiation. The receivedlow and high energy imaging radiation then may be processed at computersystem 250, described further below, to generate first and second setsof image data using suitable software and to produce real-time x-rayimages having enhanced definition for radio-opaque objects and softtissue. In one embodiment, first and second x-ray detectors areenergy-binning photon counting detectors.

In an alternative embodiment, first and second x-ray sources 206 and 208may each alternate between low energy and high energy emission modesduring respective periodic imaging cycles. In one embodiment, first andsecond x-ray sources 206 and 208 may be in-phase with each other (i.e.,both emitting at low energy, then both emitting at high energy, etc.),while for another embodiment, first and second x-ray sources 206 and 208may be out of phase with each other (i.e., one emitting at low energywhile the other emits at high energy).

In accordance with another aspect of the present invention, thereal-time imaging system of IGRT system 200 may be configured to providemulti-energy stereoscopic tomosynthesis images. X-ray tomosynthesisrefers to the process of acquiring a number of two-dimensional x-rayprojection images of a target volume using x-rays that are incident uponthe target volume at a respective number of different angles, followedby the mathematical processing of the two-dimensional x-ray projectionimages to yield a set of one or more tomosynthetic reconstructed imagesrepresentative of one or more respective slices of the target volume,wherein the number of x-ray projection images is less than that in a setthat would be required for CT image reconstruction, and/or the number orrange of incident radiation angles is less than would be used in a CTimaging procedure.

In this embodiment, first and second x-ray source-detector pairs(206/210, 208/212) are positioned to acquire tomosynthesis projectionimages over first and second non-overlapping projection angle ranges.First and second sets of tomosynthesis projection images of the targetvolume are acquired at distinct first and second x-ray energy levels,respectively (e.g., 80 kV and 140 kV), using the respective first andsecond x-ray tomosynthesis source-detector pairs.

The first and second sets of tomosynthesis projection images then areprocessed to generate respective first and second tomosynthesisreconstructed image sets of the target volume. Any of a variety ofdifferent tomosynthesis reconstruction algorithms may be used including,but not limited to, filtered backprojection (FBP), matrix inversiontomosynthesis (MITS), maximum likelihood expectation maximization(MLEM), and iterative ordered-subset convex (OSC) algorithms based on amaximum-likelihood models.

The first and second tomosynthesis reconstructed image sets then areprocessed in conjunction with each other on a locationwise basis (e.g.,voxelwise basis) within the target volume to generate a dual-energyprocessed image set. In one embodiment, the processing of the first andsecond tomosynthesis reconstructed image sets comprises registration(either by a known physical transformation between the imagingcoordinate spaces or by image-based registration) and combinationprocessing and/or other decomposition into soft-tissue and bone imagecomponents. Treatment radiation is delivered to the treatment targetwithin the target volume based at least in part on the dual-energyprocessed image set.

Still referring to FIG. 2, computer system 250 is integrated with and/orcoupled to IGRT system 200 using one or more buses, networks, or othercommunications systems 260, including wired and/or wirelesscommunications systems, and being capable in conjunction therewith ofimplementing the methods of one or more of the preferred embodiments.Methods of image guided radiation treatment in accordance with one ormore of the preferred embodiments may be implemented in machine-readablecode (i.e., software or computer program product) and performed oncomputer systems such as, but not limited to, computer system 250.Computer system 250 includes central processing unit (CPU) 251 havingmicroprocessor 252, random access memory 253, and nonvolatile memory 254(e.g. electromechanical hard drive, solid state drive), display monitor255, mouse 261, keyboard 263, and other I/O devices 256 capable ofreading and writing data and instructions from machine-readable media258 such as tape, compact disk (CD), digital versatile disk (DVD),blu-ray disk (BD), and so forth. In addition, computer system 250 may beconnected via one or more buses, networks, or other communicationssystems 260 to other computers and devices, such as may exist on anetwork of such devices, e.g., Internet 259. Software to control theimage guided radiation treatment steps described herein may beimplemented as a program product and stored on a tangible storage devicesuch as machine-readable medium 258, external nonvolatile memory device262, cloud storage, or other tangible storage medium.

Referring now to FIGS. 3A and 3B, an x-ray source filter for generatingmulti-energy x-ray images in accordance with the present invention isdescribed. FIGS. 3A and 3B are, respectively, a side view and a planview of x-ray source filter 300. X-ray source filter 300 includes motor302, controller 304, shaft 306, and filter material 308. X-ray sourcefilter 300 may be coupled to an x-ray source, e.g., x-ray source 106 ofFIG. 1 as illustrated or x-ray sources 108, 206, 208, or any additionalx-ray sources provided.

Motor 302 is configured to turn shaft 306 in response to commands fromcontroller 304. Controller 304 communicates with computer system 250(see FIG. 2) which determines desired motor characteristics, such aspower and speed, using suitable software. Computer system 250 thentransmits the desired motor characteristics to controller 304 using awired or wireless communication link.

Shaft 306 is coupled to motor 302 and filter material 308 such thatfilter material 308 rotates when motor 302 turns shaft 306. Filtermaterial 308 is made of at least two materials suitable for filteringx-ray radiation and is configured to filter imaging radiation emittedfrom x-ray source 106. Filter material 308 includes first filtermaterial 310 and second filter material 312, wherein first filtermaterial 310 comprises different material from second filter material312. In a preferred embodiment, first filter material 310 includesaluminum (Al) and second filter material 312 includes copper (Cu).

In operation, x-ray source 106 emits imaging radiation at a first energylevel. The x-rays travel through first filter material 310 which filtersthe imaging radiation such that low energy radiation exits first filtermaterial 310. In response to information transferred from computersystem 250, controller 304 commands motor 302 to turn shaft 306 whichrotates filter material 308 such that second filter material 312 ispositioned adjacent x-ray source 106. Then, x-ray source 106 emitsimaging radiation at the same first energy level. The radiation travelsthrough second filter material 312 which filters the imaging radiationsuch that high energy radiation exits second filter material 312. Thisprocess continues such that the energy levels of imaging radiationexiting filter material 308 alternate between low and high energies.

FIGS. 4A and 4B illustrate an alternative embodiment of an x-ray sourcefilter for generating multi-energy x-ray images. X-ray source filter 400includes motor 302, controller 304, shaft 306, and filter material 408.Motor 302, controller 304, and shaft 306 operate in the same manner asdescribed with respect to FIGS. 3A and 3B. Filter material 408 is madeof material suitable for filtering x-ray radiation, e.g., Al or Cu, andis configured to filter radiation emitted from x-ray source 106. Filtermaterial 408 includes first filter thickness 410 and second filterthickness 412, wherein first filter thickness 410 is made of the samematerial as second filter thickness 412, but has a different thickness.As such, imaging radiation emitted from x-ray source 106 has a firstenergy after traveling through first filter thickness 410 and has asecond energy after traveling through second filter thickness 412.

Referring now to FIGS. 5A, 5B, and 5C, yet another alternativeembodiment of an x-ray source filter for generating multi-energy x-rayimages is described. FIG. 5A illustrates a side view of x-ray sourcefilter 500 which includes motor 502, link 504, slot 506, and filtermaterial 508. X-ray source filter 500 is coupled to an x-ray source,e.g., x-ray source 106 of FIG. 1 as illustrated or x-ray sources 108,206, 208, or any additional x-ray sources provided.

Motor 502 is configured to slide link 504 along slot 506 in response tocommands from controller 507. Controller 304 communicates with computersystem 250 (see FIG. 2) which determines desired motor characteristics,such as power and speed, using suitable software. Computer system 250then transmits the desired motor characteristics to controller 304 usinga wired or wireless communication link.

Link 504 is coupled to motor 502 and filter material 508 such thatfilter material 508 translates linearly when motor 502 slides link 504along slot 506. In one embodiment, filter material 508 is made of atleast two materials suitable for filtering x-ray radiation and isconfigured to filter imaging radiation emitted from x-ray source 106.Filter material 508 includes first filter material 510 and second filtermaterial 512, wherein first filter material 510 comprises differentmaterial from second filter material 512. In a preferred embodiment,first filter material 510 includes aluminum (Al) and second filtermaterial 512 includes copper (Cu). In an alternative embodiment, firstfilter material 510 comprises the same material as second filtermaterial 512, but has a different thickness.

In operation, x-ray source 106 emits imaging radiation at a first energylevel when filter material 508 is in a first position as illustrated inFIGS. 5A and 5B. The imaging radiation travels through first filtermaterial 510 which filters the imaging radiation such that low energyradiation exits first filter material 510. In response to informationtransferred from computer system 250, controller 507 commands motor 502to slide link 504 along slot 506 which translates filter material 508 toa second position such that second filter material 512 is positionedadjacent x-ray source 106 as illustrated in FIG. 5C. Then, x-ray source106 emits imaging radiation at the same first energy level. Theradiation travels through second filter material 512 which filters theimaging radiation such that high energy radiation exits second filtermaterial 512. This process continues such that the energy levels ofimaging radiation exiting filter material 508 alternate between low andhigh x-ray energies.

As a further alternative, an x-ray source filter may be disposed on ahinge, so that the filter may alternatingly be disposed or removed fromthe path of imaging radiation emitted by the x-ray source.

In FIGS. 3A-5C, although x-ray source filters 300, 400, and 500illustratively are separate from x-ray source 106, the filters may beintegrated into a common housing with an x-ray source without departingfrom the scope of the present invention.

Referring now to FIGS. 6A and 6B, an alternative embodiment is describedin which a filter is selectively placed before the detector to filterimaging radiation that has already passed through the patient. In thisembodiment, an x-ray detector filter for generating multi-energy x-rayimages is described in which x-ray detector filter 600 includes firstfilter material 602 and second filter material 604. As seen in theexploded view of FIG. 6A and the assembled view of FIG. 6B, x-raydetector filter 600 may be disposed on an x-ray detector, e.g., x-raydetector 110 of FIG. 1 as illustrated or x-ray detector 112, 210, 212,or any additional x-ray detector provided. Alternatively, x-ray detectorfilter 600 may be integrated into a common housing with an x-raydetector.

X-ray detector filter 600 is made of material suitable for filteringx-ray radiation and is configured to filter imaging radiation, emittedfrom an x-ray source, that has passed through part or all of a patient.In one embodiment, first filter material 602 comprises differentmaterial from second filter material 604. In a preferred embodiment,first filter material 602 includes aluminum (Al) and second filtermaterial 604 includes copper (Cu). In another preferred embodiment,first filter material 602 includes no material and second filtermaterial 604 includes copper (Cu). In an alternative embodiment, firstfilter material 602 comprises the same material as second filtermaterial 604, but has a different thickness.

In operation, an x-ray source, e.g., x-ray source 106 of FIG. 1, emitsx-ray radiation 606 which travel through a patient to x-ray detectorfilter 600. X-ray radiation 606 passes through first filter material 602or second filter material 604 of x-ray detector filter 600. First filtermaterial 602 filters x-ray radiation 606 such that low energy x-rayradiation 608 exits first filter material 602 while second filtermaterial 604 filters x-ray radiation 606 such that high energy x-rayradiation 610 exits second filter material 604. Low and high energyx-ray radiations 608 and 610 are received by x-ray detector 110. X-raydetector 110 then transmits the received low and high energy x-rayradiation 608 and 610 to a suitable computer for processing the receivedradiation into first and second sets of image data and generatingmulti-energy x-ray images.

X-ray detector filter 600 may take the form of a rectangular grid asillustrated, although many patterns are possible. For example, x-raydetector filter 600 may be patterned such that its columns alternatematerial or thickness. In a preferred embodiment, first and secondfilter materials 602 and 604 are each the size of a pixel on x-raydetector 110.

FIG. 6B illustrates an assembled view of an exemplary pair of x-raydetector filters 600 and 600′ according to one embodiment of the presentinvention. X-ray detector filter 600 is disposed on x-ray detector 110and x-ray detector filter 600′ is disposed on x-ray detector 112. X-raydetector filter 600′ has a pattern complementary to the pattern of x-raydetector filter 600, for use in stereoscopic imaging. For example, thepattern of x-ray detector filter 600′ may mirror the pattern of x-raydetector filter 600 as illustrated.

In FIGS. 3A-6B, although filter materials 308, 408, 508, and x-raydetector filter 600 illustratively include two filter materials or twothicknesses, a greater number of materials and/or thicknesses, e.g.,three, four, or more, may be employed to generate x-rays at a greaternumber of energies, e.g., three, four, or more, without departing fromthe scope of the present invention.

Systems and Methods for Processing and Using Multi-Energy X-Ray Images

The present invention provides systems and methods for processingmulti-energy x-ray images and using the same in image-guided radiationtherapy (IGRT). Advantageously, x-rays generated at different energylevels may be processed to generate enhanced image data withpositive/negative weight factors from the x-ray images. The enhancedx-ray images may be used for superior target tracking and positioning inIGRT.

FIG. 7 is a schematic view of an exemplary IGRT system for use withmulti-energy x-ray images in accordance with the principles of thepresent invention. IGRT system 700 may include diagnostic imaging system701, diagnostic image processor 702, treatment planning system 703,treatment planning library 703-A, system processor 704, memory 705,treatment and imaging system 706, and non-x-ray based position sensingsystem 710.

Diagnostic imaging system 701 is configured to generate treatmentplanning images of a region of interest within a patient. Diagnosticimaging system 701 may be a high precision volumetric imaging systemsuch as a computed tomography (CT) system or a nuclear magneticresonance imaging (MM) system.

Images generated by diagnostic imaging system 701 may be processed toenhance image features by diagnostic image processor 702 using digitalenhancement techniques known in the art. Diagnostic image processor 702may process the images to render digitally reconstructed radiographs(DRRs) using techniques known in the art. The processed images fromdiagnostic image processor 702 may be stored in treatment planninglibrary 703-A within treatment planning system 703. Treatment planninglibrary 703-A may be any kind of digital storage medium such as, forexample, magnetic or solid state media capable of storing digital x-rayimages. Treatment planning system 703 may be configured to render 3-Ddiagnostic images and one or more treatment plans, which treatment plansmay include the spatial relationship between a radiation treatment x-raysource and the region of interest during a prospective IGRT procedure.Treatment planning system 703 is coupled to system processor 704 whichmay be any type of general purpose or special purpose processing devicecapable of executing instructions and operating on image data and otherdata, and of commanding an IGRT system, such as treatment and imagingsystem 706. System processor 704 may include memory 705, which may beany type of memory capable of storing data and instructions foroperating IGRT system 700. System processor 704 may be configured toprocess first and second sets of image data received from real-timemulti-energy imaging system 709 to generate an enhanced image of part orall of the patient and to direct treatment delivery system 708 based oninformation obtained from the enhanced image, as described below.

Treatment and imaging system 706 includes controller 707 coupled totreatment delivery system 708 and real-time multi-energy imaging system709. Controller 707 may be configured to coordinate the operations oftreatment delivery system 708 and real-time multi-energy imaging system709 in response to commands from system processor 704. Treatmentdelivery system 708 includes a radiation source configured to generatetreatment radiation beams, e.g., x-ray, electron, or proton beams, basedon the commands, e.g., a programmed routine, from system processor 704.In one embodiment, treatment delivery system 708 includes articulatedrobot arm 102, e.g., a six degree-of-freedom robot arm, MV radiationsource 104, and treatment table 114 of FIG. 1. In another embodiment,treatment delivery system includes a gantry configured to position theradiation source.

Treatment delivery system 708 may further include a collimatorconfigured to collimate the treatment radiation beams. The collimatormay be a fixed collimator, a variable aperture collimator having anaperture, or a multileaf collimator having leaves. In one embodiment,the variable aperture collimator approximates a circular field. Systemprocessor 704 may direct treatment delivery system 708 to set theaperture based on information obtained from the enhanced image, tochange the diameter of the circular field, to move the leaves of themultileaf collimator, to move the variable aperture collimator, to movethe multileaf collimator, and/or to position the patient.

System processor 704 may direct treatment delivery system 708 based onother information obtained from the enhanced image. Such information mayinclude the position of the target within a patient, the position of askeletal structure within the patient, the position of a soft tissuewithin the patient, and/or the position of fiducials within the patientand preferably within the target. System processor 704 may directtreatment delivery system 708 to position the radiation source, positionthe patient using, for example, treatment table 114, and/or enable ordisable the treatment radiation beams based on the information.

Real-time multi-energy imaging system 709 is configured to generate two,three, four, or more sets of image data at two, three, four, or moreenergy levels for generating multi-energy x-ray images of a region ofinterest that includes the target, the skeletal structure, the softtissue, and/or fiducials in real-time during an IGRT procedure. The setsof image data may be volumetric image data, such as tomosynthetic imagedata or CT image data, or a plurality of two-dimensional projectionimage data. Real-time multi-energy imaging system 709 may includeapparatus described above with respect to FIGS. 2-6B for generatingmulti-energy x-ray images such as x-ray sources 106, 108, 206, 208,x-ray detectors 110, 112, 210, 212, x-ray source filters 300, 400, 500,and/or x-ray detector filter 600, or may include apparatus known to oneof ordinary skill in the art for generating multi-energy x-ray imagessuch as a sandwich-type x-ray detector. Real-time multi-energy x-rayimages acquired from real-time multi-energy imaging system 709 may beprocessed by system processor 704 to enhance image features which mayimprove similarity measures between the pre-treatment and in-treatmentimages.

Real-time multi-energy imaging system 709 may include an x-ray sourcearray having a plurality of emission spots. A first emission spot of theplurality of emission spots may be configured to emit imaging radiationat the first energy level, and optionally subsequently at the secondenergy level, and a second emission spot of the plurality of emissionspots that may be configured to emit imaging radiation at the secondenergy level, and optionally subsequently at the first energy level. Theplurality of emission spots may be configured to emit imaging radiationat the first energy level and to subsequently emit imaging radiation atthe second energy level. The multi-energy imaging system may furtherinclude a second x-ray source array having a plurality of emission spotsconfigured to emit imaging radiation in-phase or out of phase with theemission spots of the first x-ray source array. The plurality ofemission spots of the first x-ray source array may be configured to emitimaging radiation at the first energy level, and optionally subsequentlyat the second energy level, and the plurality of emission spots of thesecond x-ray source array may be configured to emit imaging radiation atthe second energy level, and optionally subsequently at the first energylevel. The plurality of emission spots of the first x-ray source arrayand the plurality of emission spots of the second x-ray source arrayalso may be configured to emit imaging radiation at the first energylevel and to subsequently emit imaging radiation at the second energylevel.

System processor 704 is configured to enhance image features usingdigital enhancement techniques known in the art. In a preferredembodiment, system processor 704 is configured to combine x-ray imagesgenerated at two or more energies to provide multi-energy x-ray images.For example, system processor 704 may combine x-rays generated within alow energy range (e.g., ˜50-100 kV) with x-rays generated within a highx-ray energy range (e.g., 100 kV-6 MV, preferably 100-150 kV) to provideenhanced images of objects of interest, e.g., target, soft tissue, andradio-opaque objects such as skeletal structures, fiducials, andcontrast agents. System processor 704 may process image data wherein theenhanced image comprises a weighted combination of the first and secondsets of image data. The enhanced image may comprise a weightedcombination in logarithmic space of the first and second sets of imagedata and/or a spatially varying weighted combination of the first andsecond sets of image data. The enhanced image may also be a weightedsubtraction in logarithmic space of the first and second sets of imagedata. The enhanced image may be an x-ray image displaying primarily softtissue without radio-opaque objects such as skeletal structures,fiducials, and contrast agents. Alternatively, the enhanced image may bean x-ray image displaying primarily radio-opaque objects without softtissue.

FIGS. 8A and 8B are exemplary x-ray images that may be generated byreal-time multi-energy imaging system 709 and FIGS. 8C and 8D are x-raysimages that may be processed by system processor 704, available fromhttp://en.wikibooks.org/wiki/Basic_Physics_of_Nuclear_Medicine/Dual-Energy_Absorptiometry.FIG. 8A is a chest x-ray generated at 56 kV, i.e., a low energy x-rayimage. FIG. 8B is a chest x-ray generated at 120 kV, i.e., a high energyx-ray image, using a copper filter. FIG. 8C is a processed chest x-raydisplaying soft tissue and having the skeletal structures subtractedout. FIG. 8D is a processed chest x-ray displaying skeletal structuresand having the soft tissue subtracted out.

Referring back to FIG. 7, registration of the pre-treatment andin-treatment images may be performed by system processor 704 on imagedata sent to system processor 704 from treatment planning system 703 andreal-time imaging system 709. The registration of the pre-treatment andin-treatment x-ray images may include calculation of in-planetranslations, in-plane rotation and out-of-plane rotation, as is knownin the art.

Non-x-ray based position sensing system 710 is configured to generate arespiratory motion model of a patient. Non-x-ray based position sensingsystem 710 senses position and/or movement of external marker(s)strategically affixed to the patient, and/or senses position and/ormovement of the patient skin surface itself, using one or more methodsthat do not involve ionizing radiation, such as optically based orultrasonically based methods. Non x-ray based position sensing system710 may include external markers affixed in some manner to a patient'schest which move in response to respiration (other mechanisms formonitoring respiration may be used), and include a mono or stereoscopicx-ray imaging system which can precisely determine target location.System 710 correlates motion of the external markers with target motion,as determined from (for example) the mono or stereoscopic x-rayprojections. Non x-ray based position sensing system 710 permits systemprocessor 704 to monitor external marker motion, use the correlationmodel to precisely predict where the target will be located in real time(e.g., ˜60 Hz), and direct the treatment beam to the target. Astreatment of the moving target progresses, additional x-ray images maybe obtained and used to verify and update the correlation model. In oneexample, non-x-ray position sensing system 710 is similar to SYNCHRONY®Respiratory Tracking System available from Accuray Incorporated ofSunnyvale, Calif.

FIG. 9 illustrates a method for processing multi-energy x-ray image datain IGRT to enhance image quality in interventional images by exploitingbeam hardening effects using High Dynamic Range (HDR) x-ray imaging.With reference to FIG. 7, method 900 may include determining an optimalenergy level having the highest definition or differentiation abilityfor an object of interest, e.g., soft tissue, for imaging the objectwith real-time multi-energy imaging system 709 using a phantom withsystem processor 704, at step 910.

FIG. 10 is a plot illustrating a change in x-ray spectra after traversalthrough a filter for determining the optimal energy level. The plotshows count versus energy of x-rays that traversed aluminum plateshaving varied thickness. X-ray energy level 1000 is the determinedoptimal energy level for the selected object of interest.

Referring back to FIG. 9, at step 920, a first set of image data isgenerated using imaging radiation at the optimal energy level withreal-time multi-energy imaging system 709. At step 930, the energy ofthe imaging radiation is adjusted with controller 707 and a second setof image data is generated using imaging radiation at the adjustedenergy level with real-time multi-energy imaging system 709. Then, atstep 940, the second set of image data generated at the adjusted energylevel is processed with the first set of image data generated at theoptimal energy level using system processor 704 to generate a processedHDR x-ray image. Advantageously, the processed HDR x-ray image isgenerated from a combination of x-rays generated over a wide energyspectrum.

FIG. 11 illustrates a method for registering multi-energy x-ray imagesin IGRT in accordance with one aspect of the present invention. Withreference to FIG. 7, method 1100 may include, in a treatment planningphase, generating treatment planning images, e.g., a CT volume, withdiagnostic imaging system 701 at step 1110. Then, at step 1120, thetreatment planning images are pre-processed with diagnostic imageprocessor 702 to generate digitally reconstructed radiographs (DRRs). Ina treatment delivery phase, real-time multi-energy x-ray image data isgenerated with real-time multi-energy imaging system 709 at step 1130.For example, a first set of image data may be generated using imagingradiation at a first energy level and a second set of image data may begenerated using imaging radiation at a second energy level. At step1140, the real-time multi-energy x-ray image data is processed withsystem processor 704 to extract real-time x-ray image features and tooptionally scale the real-time multi-energy x-ray image data to correctfor different imaging geometries between diagnostic imaging system 701and real-time multi-energy imaging system 709. Processing may includecombining image data from x-rays generated at different energy levels.At step 1150, the real-time multi-energy x-ray image data is registeredwith corresponding DRRs with system processor 704 to obtain aregistration result. Optionally, at step 1160, the real-timemulti-energy x-ray images are tracked against the DRRs with systemprocessor 704 to determine one of interest/patient movement and/orposition. Beneficially, tracking the object of interest withmulti-energy x-rays images can provide precise target location duringIGRT for enhanced accuracy of treatment radiation emission fromtreatment delivery system 708 to the target.

FIG. 12 illustrates a method for tracking target(s), skeletalstructure(s), soft tissue(s), contrast agent(s), fiducial(s), patientmovement and/or position using multi-energy x-ray images in IGRT inaccordance with another aspect of the present invention. With referenceto FIG. 7, method 1200 may include, in a treatment delivery phase,generating real-time multi-energy x-ray image data with real-timemulti-energy imaging system 709 at step 1210. For example, a first setof image data may be generated using imaging radiation at a first energylevel and a second set of image data may be generated using imagingradiation at a second energy level. Then, at step 1220, the real-timemulti-energy x-ray image data is processed with system processor 704 tocombine image data from x-rays generated at different energy levels. Ina preferred embodiment, system processor 704 processes the multi-energyx-ray image data to combine negative weight factor to subtractradio-opaque objects to generate primarily soft tissue x-ray images andto subtract soft tissues to generate primarily radio-opaque x-rayimages. At step 1230, real-time soft-tissue x-ray images are registeredwith corresponding DRRs with system processor 704 to obtain a softtissue registration result and real-time radio-opaque object x-rayimages are registered with corresponding DRRs with system processor 704to obtain a radio-opaque registration result. The soft tissue imagesand/or the radio-opaque object images may be tracked against the DRRswith system processor 704 to determine target/skeletal structure/softtissue/fiducial/contrast agent/patient movement and/or position at steps1240 and 1245.

Advantageously, tracking the target, e.g., a tumor, with soft tissuex-ray images may provide images in which the target is not obscured byoverlapping bones, thereby permitting precise determination of thetarget location during IGRT. In the case where a target is partially orcompletely blocked by skeletal structures using conventional x-rayimages, the soft tissue x-ray images of the present invention mayprovide a clear target image, allowing for superior target localizationand tracking. Moreover, skeletal x-ray images may be used for enhancedpatient positioning, e.g., for initial spine alignment, and to trackskeletal motion and detect patient shift. Because the techniques of thepresent invention provide enhanced target/patient movement and/orpositioning, the accuracy of treatment radiation emission from treatmentdelivery system 708 to the target is expected to be superior topreviously-known techniques.

FIG. 13 illustrates a method for target tracking using a motion-edgeartifact in IGRT in accordance with yet another aspect of the presentinvention. With reference to FIG. 7, method 1300 may include, in atreatment delivery phase, generating a first set of image data withreal-time multi-energy imaging system 709 at step 1310.

FIG. 14A is an illustration of an x-ray image of target 1400 at firstposition 1401 generated using the first set of image data.

Referring back to FIG. 13, at step 1320, a second set of image data isgenerated with multi-energy imaging system 709. In a preferredembodiment, the first set of image data is generated using radiation ata first energy level and the second set of image data is generated usingradiation at a second energy level.

FIG. 14B is an illustration of an x-ray image of target 1400 at secondposition 1402 generated using second x-ray image data. Illustratively,target 1400 has moved along the Axis of Motion from first position 1401to second position 1402. Additionally, the Axis of Motion of the targetmay be estimated from a 4-D CT scan or a 3-D CT inhale/exhale pairduring treatment planning with treatment planning system 703.

Again, referring back to FIG. 13, at step 1330, the first and secondsets of image data are processed with system processor 704 to performweighted combination of an overlapping portion of the first and secondsets of image data to generate a motion-edge artifact image. Motionartifacts are effects in imaging caused by movement of the target beingimaged and are generally seen in images as blurring and/or streaking.The size of the motion artifact may be predicted from a 4-D CT scanbecause size mostly depends on the dimensions of the target and targetvelocity in a particular respiratory phase.

FIG. 14C is an illustration of the motion-edge artifact image generatedafter combining overlapping portions of the target at first position1401 and second position 1402. The non-overlapping portions of thetarget at first and second positions 1401 and 1402 are motion-edgeartifacts. Trailing edge artifact 1403 is the area in the x-ray imagethat was imaged at first position 1401, but not imaged at secondposition 1402. Leading edge artifact 1404 is the area in the x-ray imagethat was imaged at second position 1402, but not imaged at firstposition 1401.

Referring yet again back to FIG. 13, at step 1340, the movement and/orposition of the target is tracked with system processor 704 using themotion-edge artifact image which may be inputted into suitable software.In a preferred embodiment, method 1300 is used to track the target,e.g., a tumor. In an alternative embodiment, method 1300 may be used totrack non-target structures such as a rib or diaphragm. Advantageously,the methods of the present invention provide enhanced targetlocalization using traditionally unwanted image artifacts as anadditional constraint for target localization in IGRT.

FIG. 15 illustrates a method for target/soft tissue/skeletalstructure/contrast agent/fiducial tracking during respiration usingmulti-energy x-ray images in IGRT in accordance with the presentinvention. With reference to FIG. 7, method 1500 may include, in atreatment delivery phase, generating a first set of image data at aposition in the patient's respiratory cycle using imaging radiation at afirst energy level with real-time multi-energy imaging system 709 atstep 1510. At step 1520, a second set of image data is generated at thesame position in the patient's subsequent respiratory cycle usingimaging radiation at a second energy level with multi-energy imagingsystem 709. The first and second sets of image data are processed withsystem processor 704 to predict the target location at differentpositions in the respiratory cycle at step 1530. System processor 704may communicate with non-x-ray based position sensing system 710 toenhance the target location prediction based on a respiratory motionmodel generated by non-x-ray based position sensing system 710. At step1540, the movement and/or position of the object of interest and/orpatient are tracked with system processor 704 using the first and secondsets of image data and compared to the predicted target location.

In a preferred embodiment, the first energy level is a high energy levelsuch that the first set of image data includes data having greater softtissue attenuation and the second energy level is a low energy levelsuch that the second set of image data includes data having greaterskeletal structure attenuation. Because soft tissue displacements mayvary significantly between respiration cycles due to factors such astissue hysteresis and variations in alveolar recruitment, skeletalstructures may correlate well with the external markers of non-x-raybased position sensing system 710. As such, the first and second sets ofimage data may be processed to combine the data so that the soft tissueimage data is the most recently acquired data and may be used for targettracking and to update the model from non-x-ray based position sensingsystem 710.

It will be apparent from the foregoing description to one skilled in theart that aspects of the present invention may be embodied, at least inpart, in software. That is, referring to FIG. 7, the techniques may becarried out in a computer system or other data processing system inresponse to its processor, such as system processor 704, executingsequences of instructions contained in a memory, such as memory 705. Invarious embodiments, hardware circuitry may be used in combination withsoftware instructions to implement the present invention. Thus, thetechniques are not limited to any specific combination of hardwarecircuitry and software or to any particular source for the instructionsexecuted by the data processing system. In addition, throughout thisdescription, various functions and operations may be described as beingperformed by or caused by software code to simplify description.However, those skilled in the art will recognize what is meant by suchexpressions is that the functions result from execution of the code by aprocessor or controller, such as system processor 704 or controller 707.

A machine-readable medium can be used to store software and data which,when executed by a data processing system, causes the system to performvarious methods of the present invention. This executable software anddata may be stored in various places including, for example, memory 705and treatment planning library 703-A or any other device that is capableof storing software programs and/or data.

A machine-readable medium includes any mechanism that provides (i.e.,stores and/or transmits) information in a form accessible by a machine(e.g., a computer, network device, personal digital assistant,manufacturing tool, any device with a set of one or more processors,etc.). For example, a machine-readable medium includesrecordable/non-recordable media (e.g., read only memory (ROM); randomaccess memory (RAM); magnetic disk storage media; optical storage media;flash memory devices; etc.), as well as electrical, optical, acousticalor other forms of propagated signals (e.g., carrier waves, infraredsignals, digital signals, etc.).

While various illustrative embodiments of the invention are describedabove, it will be apparent to one skilled in the art that variouschanges and modifications may be made therein without departing from theinvention. The appended claims are intended to cover all such changesand modifications that fall within the true scope of the invention.

What is claimed is:
 1. A method of image-guided radiation, the methodcomprising: acquiring a pre-treatment image of a target of a patient;generating a first set of image data of part or all of the target usingimaging radiation at a first energy level and a second set of image dataof part or all of the target using imaging radiation at a second energylevel; processing the first and second sets of image data to generate anenhanced image, wherein the enhanced image comprises a combination ofthe first and second sets of image data, wherein part or all of theimage data comprises the target; registering the enhanced image with thepre-treatment image to obtain a registration result; tracking the targetusing the registration result to generate tracking information; anddirecting treatment delivery to the target based on the trackinginformation obtained from the enhanced image.
 2. The method of claim 1,wherein the pre-treatment image comprises a computed tomography (CT)image.
 3. The method of claim 1, wherein the tracking informationobtained from the enhanced image comprises a position of the target. 4.The method of claim 1, wherein directing treatment delivery comprises:generating a treatment delivery beam using a treatment delivery systemcomprising a radiation source; and directing the treatment delivery beamto the target based on the tracking information.
 5. The method of claim4, wherein directing treatment delivery comprises further comprisesadjusting an angle at which the treatment delivery beam is directed tothe target.
 6. The method of claim 1, wherein directing treatmentdelivery comprises moving a treatment couch to position the target ofthe patient.
 7. The method of claim 1, wherein directing treatmentdelivery comprises adjusting a collimator to direct a treatment deliverybeam to the target based on the tracking information.
 8. A system,comprising: a radiation source; a processor, operatively coupled withthe radiation source, to: generate a first set of image data of part orall of a target of a patient using imaging radiation at a first energylevel and a second set of image data of part or all of the target usingimaging radiation at a second energy level; process the first and secondsets of image data to generate an enhanced image, wherein the enhancedimage comprises a combination of the first and second sets of imagedata, wherein part or all of the image data comprises the target;register the enhanced image with a pre-treatment image of the target toobtain a registration result; track the target using the registrationresult to generate tracking information; and direct treatment delivery,using the radiation source, to the target based on the trackinginformation obtained from the enhanced image.
 9. The system of claim 8,wherein the pre-treatment image comprises a computed tomography (CT)image.
 10. The system of claim 8, further comprising: a diagnosticimaging system to acquire a pre-treatment image.
 11. The system of claim8, wherein to direct treatment delivery, the processor is to: cause theradiation source to generate a treatment delivery beam; and direct thetreatment delivery beam to the target based on the tracking information.12. The system of claim 8, further comprising a treatment couch, whereinthe processor is operatively coupled with treatment couch to positionthe target of the patient based on the tracking information.
 13. Thesystem of claim 8, further comprising a collimator, wherein theprocessor is operatively coupled with the collimator to adjust thecollimator to direct the treatment delivery beam to the target based onthe tracking information.
 14. The system of claim 8, wherein the firstenergy level is an energy level selected within the range of 50-100 kVand the second energy level is an energy level selected within the rangeof 100-150 kV.
 15. A non-transitory machine readable medium that, whenexecuted by a processor, causes the processor to: generate a first setof image data of part or all of a target of a patient using imagingradiation at a first energy level and a second set of image data of partor all of the target using imaging radiation at a second energy level;process the first and second sets of image data to generate an enhancedimage, wherein the enhanced image comprises a combination of the firstand second sets of image data, wherein part or all of the image datacomprises the target; register, by the processor, the enhanced imagewith a pre-treatment image of the target to obtain a registrationresult; track the target using the registration result to generatetracking information; and direct treatment delivery, using a radiationsource, to the target based on the tracking information obtained fromthe enhanced image.
 16. The non-transitory machine readable medium ofclaim 15, wherein the pre-treatment image comprises a computedtomography (CT) image.
 17. The non-transitory machine readable medium ofclaim 15, wherein the processor is further track information obtainedfrom the enhanced image comprises a position of the target.
 18. Thenon-transitory machine readable medium of claim 15, wherein to directtreatment delivery, the processor is to: generate a treatment deliverybeam using a treatment delivery system comprising the radiation source;and direct the treatment delivery beam to the target based on thetracking information.
 19. The non-transitory machine readable medium ofclaim 18, wherein to direct treatment delivery, the processor further toadjust an angle at which the treatment delivery beam is directed to thetarget.
 20. The non-transitory machine readable medium of claim 15,wherein to direct treatment delivery, the processor is to move atreatment couch to position the target of the patient.
 21. Thenon-transitory machine readable medium of claim 15, wherein to directtreatment delivery, the processor is to adjust a collimator to directthe treatment delivery beam to the target based on the trackinginformation.
 22. The non-transitory machine readable medium of claim 15,wherein the first energy level is an energy level selected within therange of 50-100 kV and the second energy level is an energy levelselected within the range of 100-150 kV.